1. Field of the Invention
The invention relates to the purification of gases in a pressure swing adsorption system. More particularly, it relates to improvements in the processing cycle and system enabling improved performance to be achieved.
2. Description of the Prior Art
The pressure swing adsorption (PSA) process provides a commercially desirable technique for separating and purifying at least one component of a feed gas mixture containing said component and at least one selectively adsorbable component. Adsorption occurs in an adsorbent bed at a higher adsorption pressure, with the selectively adsorbable component thereafter being desorbed by reducing the adsorbent bed pressure to a lower desorption pressure. The carrying out of the PSA process in multi-bed systems is illustrated by the Wagner patent, U.S. Pat. No. 3,430,418, relating to a system having at least four beds. As is generally known and described in this patent, the PSA process is commonly carried out, on a cyclic, in a processing sequence that includes, in each bed, (1) higher pressure adsorption with release of product effluent from the product end of the bed, (2) cocurrent depressurization to intermediate pressure with release of void space gas from the product end thereof, (3) countercurrent depressurization to a lower desorption pressure, (4) purge and (5) repressurization. The void space gas released during the cocurrent depressurization step is commonly employed for pressure equalization purposes and to provide purge gas to a bed at its lower desorption pressure.
In a variation of said PSA processing described above with reference to systems having four or more absorbent beds, a conventional three bed system was devised for use in the separation and recovery of air and other such separations. This system was based on the increasing pressure adsorption step described in the McCombs patent, U.S. Pat. No. 3,738,087. In one embodiment thereof, air is added to an adsorbent bed for the repressurization thereof, with nitrogen being selectively adsorbed and with oxygen being discharged from the product end of the bed at rates such that the bed pressure increases to upper adsorption pressure. A PSA cycle incorporating said increasing pressure adsorption step includes (1) said increasing pressure adsorption step, (2) cocurrent depressurization to intermediate pressure with release of void space gas from the product end thereof, (3) countercurrent depressurization to a lower desorption pressure, (4) purge and (5) partial repressurization. The void space gas released during the cocurrent depressurization step is employed, in this embodiment, for passage to other beds in the system in a pressure equalization-provide purge - pressure equalization sequence. This latter cycle makes unnecessary a constant pressure adsorption step as employed in the Wagner cycle. This enables more time for bed regeneration, i.e. countercurrent depressurization and purge, within a given cycle time so as to enable greater productivity and recovery and/or purity to be obtained from a given system, particularly in systems designed for relatively short overall cycle time operation.
Using such a three bed system with each bed containing commercial 13X, 8.times.12 bead form, molecular sieve in air separation operations, an oxygen recovery of 48% and a productivity (BSF) of 4,000 lb. 13 X molecular sieve per one ton per day (TPD) of oxygen have been obtained. Said recovery is defined as the percent or volume fraction of the feed air oxygen removed from the feed stream and delivered as oxygen product. Productivity is defined as the pounds of molecular sieve required to generate 1 TPD of contained oxygen. The recovery and productivity values referred to above were obtained on the basis of a 180 second total cycle time for the 3-bed PSA system, with feed air being introduced at a maximum pressure of 40 psig, with product being discharged at 20 psig.
While such standard 3-bed system is desirable for various commercial applications, there is, nevertheless, a desire in the art to improve product recovery and productivity. Difficulties have been encountered, however, in achieving such objectives. Thus, the total cycle time had to be reduced to less than said 180 seconds to yield a significant BSF reduction (productivity increase) compared to said standard 3-bed operation. However, reductions in individual step times, i.e. the purge and pressure equalization steps, are limited by gas velocity and bed fluidization limits, or by applicable cycle performance standards. Such limitations prevent the achieving of substantial cycle time reductions only by means of reductions in the duration of the individual cycle steps. With respect to the standard 4-bed system, on the other hand, the addition of a fifth adsorbent bed to increase single bed capacity limits by means of standard cycling techniques applicable to said systems would necessarily result in an increase in total cycle times and in the BSF values for any given application. Such an increase in BSF value would compromise any potential increase in productive capacity derived from an increase in the number of vessels employed in the PSA system. ln addition, size limitations on PSA-oxygen adsorbent beds limit the maximum capacity of a single PSA train, so that the development of means to reduce the BSF would be required to increase the maximum capacity limits of such a single PSA train. There remains in the art, therefore, a need to develop improvements in the PSA art enabling reductions in BSF and increased single train capacity to be achieved. Such improvements advantageously would enable the overall cycle time to be reduced, while enabling sufficient time to complete each individual cycle time without degradation of product purity or recovery.
It is an object of the invention, therefore, to provide an improved PSA process and system.
It is another object of the invention to provide a PSA process and system for the enhanced separation and recovery of oxygen from air.
It is another object of the invention to provide a PSA process and system enabling overall cycle times to be minimized while enabling sufficient time to complete each individual cycle step without degradation in product purity or recovery.